This application relates generally to circuits for protecting transistors from adverse applied voltages. In particular, the application relates to protection of bipolar transistors from excess reverse Vbe voltages, including those resulting in μ-degradation over time.
The phenomenon of μ-degradation is a response of a bipolar transistor to a stress. A bipolar transistor is a three-terminal amplifying device, whose terminals are referred to as the base, the collector and the emitter. There are junctions between the materials of which the base and collector are formed and between the materials of which the base and emitter are formed. The amount of gain, i.e. how large is the amplifying factor of the device, is called μ, hence degradation of the gain in response to a stress is called μ-degradation.
The phenomenon of μ-degradation occurs when a bipolar transistor is stressed by a reverse Vbe voltage in a range between zero volts and the reverse breakdown voltage of the base-emitter junction of the bipolar transistor. When reverse-biased with a voltage between zero and a voltage at which reverse breakdown, i.e., zener breakdown, occurs, a transistor junction will exhibit a small reverse leakage current. This reverse leakage current or reverse conduction causes hot carrier induced oxide damage to the oxide overlying the junction, resulting in μ-degradation and increased noise. Hot carrier induced oxide damage results in electron migration into the oxide, creating undesired additional current paths, reducing breakdown voltage and causing the increase in noise. The range of voltages for which μ-degradation occurs and the range of voltages for which operation is considered to be normal is dependent on the process used to manufacture the bipolar transistor.
Although many circuit designs do not produce voltages across the base-emitter junction, Vbe, that result in μ-degradation, there are other circuits for which this problem will occur during normal circuit operation. One example of such a circuit is the video multiplexer circuit shown in FIG. 1A. Another example is shown in FIG. 1B. Although aspects of embodiments of the invention will be described as they apply to the circuit of FIG. 1A, and both FIGS. 1A and 1B are feedback connections, it will be seen that embodiments of aspects of the invention are not so limited.
The transconductance multiplexer of FIG. 1A includes a first transconductance input amplifier 101 and a second transconductance input amplifier 102 whose outputs are combined at summing junction 103 and converted to an output voltage vo by amplifier 104. The output voltage vo is fed back to inputs VNA and VNB of input transconductance amplifiers 101 and 102, respectively.
Each input transconductance amplifier 101 and 102 is a differential transistor amplifier. Conventionally, a differential transistor amplifier comprises a differential pair of bipolar transistors, tied at the emitters to a current source, the differential pair steering current through the collectors of the differential pair to one leg or the other of the circuit depending on the differential input voltage. A differential transistor amplifier could also be constructed using other transistor types, such as JFETs, MOSFETs, etc. At the point in time shown, input transconductance amplifier 101 is enabled by current source 105 while input transconductance amplifier 102 is disabled by current source 106. At other points in time, input transconductance amplifier 101 could be disabled by current source 105 and input transconductance amplifier 102 could be enabled by current source 106, or both input transconductance amplifiers 101 and 102 could be disabled by their respective current sources 105 and 106. Because of the feedback connection, which renders the combination of amplifier 104 and enabled input transconductance amplifier 101 a voltage follower, a voltage substantially equal to input voltage va applied to terminal VPA of input transconductance amplifier 101 is produced as the output voltage vo. However, the input to the second input transconductance amplifier 102, vb, applied to input terminal VPB of disabled input transconductance amplifier 102 is independent of, and may be substantially different from, output voltage vo.
A different feedback path is active in the circuit of FIG. 1B, but the result that the inputs to at least one transconductance amplifier input stage are far apart remains the same. Indeed, in transconductance stages including a differential pair, regardless of what mechanism causes the inputs to be very different, the fact that they are different causes the problem discussed.
FIG. 2 is a schematic of one of the amplifiers 101 and 102. It can be seen from this schematic that the large voltage between the input terminals VPB and VNB of input transconductance amplifier 102 may be large enough to substantially stress the transistors comprising the differential circuit of input transconductance amplifier 102. For example, if the voltage at VN is brought to a high level, say +1.5 V for example, while the voltage at VP is brought to a low level, say −1.5 V for example, then the 3 V input is distributed across transistors Q7 and Q5 in parallel with transistors Q8 and Q6, RD, and transistors Q3 and Q1 in parallel with transistors Q4 and Q2, principally as a reverse Vbe on transistors Q7, Q6, Q3 and Q2.